There are many detectors for gases which use indicator or detector strips. These strips are exposed to the atmosphere where a suspected gas might be located. If a suspected gas is in the air, then the exposed strip will provide a change in color which is converted into gas concentration. The example that will be used below is a strip of tape which is responsive to hydrogen sulfide (H.sub.2 S). The hydrogen sulfide gas indicator strip is normally made of lead acetate which is impregnated into the fibers of the paper strip. The strip is normally formed of a porous fiber base, paper being the preferred form, typically using a strip of one quarter to one half inch in width. The strip typically is paper, and is comprised of a layer of cellulose fibers which are matted into the paper of specified thickness and width. After fabrication of the paper strip, it is impregnated with a solvent which carries the lead acetate. The solvent can be water or the like.
On exposure to H.sub.2 S, the lead acetate strip changes color. This color change is indicated by an optical densitometer. At one end of the scale, the white level provides a first data point and at the other end of the scale, the black intensity provides the other end point. Between the two end points, the scale of the optical density is defined. This yields a range which has to be correlated with other factors. The range at the black end indicates maximum exposure to H.sub.2 S. At the other gray levels between, an intermediate value of gas concentration is indicated, In all instances, the gas concentration is a variable which is best measured using the lead acetate strip paper of the present disclosure.
Measurement of H.sub.2 S concentration in the immediate vicinity is normally required in petrochemical manufacturing plants and especially at drilling rigs where wells are being drilled into formations suspected of having sour gas. Natural gas is often produced in formations having no fragrance or odor. It is known as sweet gas in that situation. Sour gas however is natural gas which is recovered in the presence of H.sub.2 S. The response to H.sub.2 S depends on the nature of the exposure. For instance, long term, low level exposure may be quite damaging or detrimental. High concentrations of H.sub.2 S can be fatal to humans, while smaller or intermediate concentrations can damage plant life, livestock and so on. The human nose responds to the strong pungent fragrance of H.sub.2 S in mild concentrations. This equipment however is needed to extend beyond the range of sensitivity of the human nose. For instance, very small or light or trace quantities have a cumulative effect also. The lead acetate tape mechanisms are installed at a number of places around a plant where H.sub.2 S is suspected of being present. It is also present in chemical test laboratory equipment and is used in copious quantities in college laboratories in qualitative test procedures. Nevertheless, it is important to measure the exposure of students and workers to the H.sub.2 S atmosphere.
Low levels are difficult to measure because they involve very little interaction with lead acetate. It is possible to concentrate the gas that is in the area by collecting more gas in a sample and increasing the flow rate of the gas so that a larger volume of gas sweeps past the lead acetate tape. As more gas is "seen" by the tape, the output data is more significant. Scale factors are thus changed to accommodate some of the limitations in the optical density variations permitted by the lead acetate tape.
The present disclosure is a system which enhances the range of scale values permitted in lead acetate tape. As noted, the tape can be speeded up or slowed down. Also as noted, the amount of air exposed to the tape or the amount of air seen in a time interval by the tape can be raised or lowered by changing the flow rate. The present disclosure however provides a marked extension of the range of the tape so that scale values can be changed significantly. Especially the tape can be made more sensitive at the low end and yet the upper end can be extended so that it does not overload i.e. become difficult to read because the black end point is saturated or overdriven. This disclosure contemplates the addition of a protective layer on top of the tape. The layer serves as an attenuation barrier so that tape exposure is reduced. The hydrogen sulfide molecules are slowed in their diffusion into the tape, thereby retarding the rate or amount or both of the chemical conversion of the lead acetate tape. In other words, the color conversion is modulated in a way to extend the range of the tape dynamically.
The present disclosure is summarized as an improved lead acetate tape. The tape comprises a paper tape formed of felted fibers of cellulose matted into a tape of specified thickness and width. A suitable solvent is used to impregnate the tape with lead acetate. The reactive agent in the solvent is left in the fibers after the solvent has evaporated. The improvement of the present disclosure contemplates the attachment of a backing to the tape of an impervious sheet membrane which prevents exposure from one place, and placement of a porous layer on the opposite or top face. The porous layer serves as a diffusion barrier of specified thickness. This either reduces or retards the time of exposure. It can readily do both, thereby enabling the response of the tape to be stretched over a greater range. This expands the optical density scale involved in reading the tape and makes the tape more responsive over a wide range of values.